1. Field of the Invention
The present invention generally relates to an orthogonal frequency division multiplexing (OFDM) communication system using a multiple input multiple output (MIMO) scheme, and more particularly. to a method for transmitting and receiving preamble sequences in the MIMO-OFDM communication system.
2. Description of the Related Art
A large amount of research is being conducted to provide users with services based on various qualities of service (QoS) at a high transmission rate in fourth-generation (4G) communication systems, which serve as next generation communication systems. In the current 4G-communication system, research is actively being conducted to support a high-speed service for ensuring mobility and QoS in broadband wireless access (BWA) communication systems such as wireless local area network (LAN) and wireless metropolitan area network (MAN) communication systems.
Research is also being conducted in an orthogonal frequency division multiplexing (OFDM) scheme useful to transmit high-speed data in a wired/wireless channel of the 4G-communication system. The OFDM scheme transmits data using a multicarrier, and is a type of multicarrier modulation (MCM) scheme for converting a serially input symbol stream to parallel signals, modulating the parallel signals into a plurality of orthogonal subcarriers, and transmitting the orthogonal subcarriers.
To provide high-speed and high-quality wireless multimedia services, the 4G-communication system needs broadband spectral resources. When the broadband spectral resources are used, the effect of fading due to multipath propagation is severe in a wireless transmission path and also the effect of frequency selective fading which occurs in a transmission band. When a high-speed wireless multimedia service is provided, the OFDM scheme robust to the frequency selective fading has a higher gain than a code division multiple access (CDMA) scheme. Therefore, the OFDM scheme is becoming actively exploited in the 4G-communication system.
A wireless communication system serving as a system for supporting a wireless communication service includes a base station (BS) and a mobile station (MS). The BS and the MS support the wireless communication service using a frame. Accordingly, the BS and the MS must acquire mutual synchronization for frame transmission and reception. For synchronization acquisition, the BS sends a synchronization signal to the MS such that the MS can know the start of the frame sent from the BS.
Then, the MS receives the synchronization signal from the BS, identifies frame timing of the BS, and demodulates a received frame on the basis of the identified frame timing. Conventionally, the synchronization signal uses a specific preamble sequence agreed to in advance between the BS and the MS.
The preamble sequence uses a low Peak to Average Power Ratio (PAPR) in the OFDM communication system. A preamble is used to perform synchronization acquisition, channel estimation, BS identification, and so on.
A preamble sequence with a low PAPR is used in the OFDM communication system. The reason for this will be described below.
Because the OFDM communication system serving as the multicarrier communication system uses a plurality of subcarriers, a transmitted signal is a sum of independent signals and therefore a difference between a maximum power value and a mean power value of time domain signal values is large. In the OFDM communication system, a PAPR value is large in a data interval and a linear interval of an amplifier provided in the OFDM system is defined on the basis of a maximum PAPR value in the data interval. Because a preamble can be transmitted at power increased by a difference with the maximum PAPR in the data interval when the PAPR of the preamble is set to a low value, the performance of channel estimation, synchronization acquisition, BS identification, and so on can be improved. Accordingly, it is important that the PAPR is lowered in the preamble interval.
When a signal transmitted from a transmitter is distorted while passing through a radio channel, a receiver receives the distorted transmitted signal. The receiver acquires time/frequency synchronization using a preamble sequence preset between the transmitter and the receiver. After channel estimation, the receiver demodulates a received signal corresponding to the distorted transmitted signal into frequency domain symbols through a fast Fourier transform (FFT). After demodulation into frequency domain symbols, the receiver decodes the frequency domain symbols into information/data through source decoding and channel decoding corresponding to channel coding applied in the transmitter.
The OFDM communication system uses a preamble sequence in all of frame timing synchronization, frequency synchronization, and channel estimation. Of course, the OFDM communication system may perform the frame timing synchronization, frequency synchronization, and channel estimation using a guard interval, a pilot subcarrier, and so on in addition to a preamble. In the case of the preamble sequence, the known symbols are transmitted in a start part of every frame or data burst. The preamble sequence is used to update estimated time/frequency/channel information through a guard interval and a pilot subcarrier of a data transmission part.
A preamble sequence and a preamble sequence generation method of the OFDM communication system using a multiple input multiple output (MIMO) scheme, i.e., multiple transmit antennas (Tx. ANTS) (e.g., NTX transmit antennas) and multiple receive antennas (Rx. ANTS) (e.g., NRX receive antennas) will be described with reference to FIG. 1, which schematically illustrates a preamble sequence mapping structure of a conventional OFDM communication system using the MIMO scheme.
FIG. 1 illustrates the preamble sequence mapping structure when BSs, for example, the first and second BSs (BS #1 and BS #2), configuring the OFDM communication system use the first and second transmit antennas (Tx. ANT #1 and Tx. ANT #2), respectively. As illustrated in FIG. 1, preamble sequences transmitted through the first and second transmit antennas of BS #1 are different from each other, and preamble sequences transmitted through the first and second transmit antennas of BS #2 are different from each other. The preamble sequences used in BS #1, i.e., the preamble sequences transmitted through the first and second transmit antennas of BS #1, are different from the preamble sequences used in BS #2, i.e., the preamble sequences transmitted through the first and second transmit antennas of BS #2.
In the cellular communication system serving as the OFDM communication system, an MS must be able to distinguish a plurality of cells. Conventionally, one BS can cover a plurality of cells. However, for convenience of explanation, it is assumed that one BS covers only one cell. As a result, the MS must be able to distinguish a plurality of BSs such that a target BS to which the MS belongs can be identified among the BSs configuring the OFDM communication system.
Accordingly, the OFDM communication system must allocate different preamble sequences between the BSs configuring the OFDM communication system and between the transmit antennas of the BSs. Each BS transmits the allocated preamble sequences through multiple transmit antennas, i.e., NTX transmit antennas. Because the different preamble sequences are transmitted through the transmit antennas of each BS as described above, an MS must have preamble sequences based on the number of transmit antennas provided in each BS. If the number of BSs configuring the OFDM communication system is M and the M BSs are provided with the NTX transmit antennas, respectively, the OFDM communication system must be provided with M×NTX preamble sequences.
In this case, the OFDM communication system must generate the M×NTX preamble sequences. When the preamble sequences with a preset length are generated, the OFDM communication system has a problem in that a maximum cross-correlation value between the preamble sequences and a PAPR increase as the number of preamble sequences increases.
Because an MS must be provided with M×NTX correlators to distinguish the M×NTX preamble sequences, the MS has a problem in that its hardware load increases. There is a problem in that a computation amount for distinguishing between BSs and synchronization acquisition using the correlators linearly increases according to the number of BSs and the number of transmit antennas.